The present invention relates to an abnormality diagnosing system of a wide-range A/F (air-fuel ratio) sensor.
A Japanese Patent Provisional Publication No. 10-169493 discloses a conventional abnormality diagnosing system of a wide-range A/F sensor. This abnormality diagnosing system is arranged to diagnose the degradation of the wide-range A/F sensor by monitoring the change of the output of the wide-range A/F sensor.
However, the conventional abnormality diagnosing system has been arranged to employ an air-fuel ratio control constant used in a normal air-fuel ratio (A/F) control even when the diagnosis of the wide-range A/F sensor is executed. The A/F control constant includes a proportional term (P-term), an integration term (I-term) and a derivative term (D-term). Therefore, the dispersion of the products of the wide-range A/F sensor and the dispersion of the air-fuel ratio at a start of the diagnosis cause a fluctuation of the time period until the output of the wide-range A/F sensor reaches a target air-fuel ratio. This fluctuation of the time period to the target value may cause a misdiagnosis as shown in FIGS. 7A and 7B and degrade the accuracy of the diagnosis. The conventional system has been arranged such that the air-fuel ratio control constant under a normal air-fuel ratio control range is set on the basis of an amplitude and a convergence of the air-fuel ratio control so as to satisfy the desired characteristics required in the normal control range as shown in FIG. 8.
It is therefore an object of the present invention to provide an improved diagnosis system for a wide-range air-fuel ratio sensor. The improved diagnosis system further accurately diagnoses whether a wide-range air-fuel ratio sensor operates correctly or not without complicating its construction.
A diagnosis system according to the present invention comprises a wide-range air-fuel ratio sensor, an air-fuel ratio feedback control section, a target air-fuel ratio switching section, a detection value variation detecting section, an abnormality diagnosing section and a diagnosis control gain setting section which are arranged as shown in FIG. 6. The wide-range air-fuel ratio sensor detects an air-fuel ratio of air-fuel mixture supplied to an engine. The air-fuel ratio feedback control section corrects an air-fuel ratio controlled object on the basis of a detection result of the wide-range air-fuel ratio sensor so as to adjust the air-fuel ratio to the engine at a target air-fuel ratio. The target air-fuel ratio switching section switches the target air-fuel ratio. The detection value variation detecting section detects variation of the detection value of the wide-range air-fuel ratio sensor. The abnormality diagnosing section diagnoses the wide-range air-fuel ratio sensor on the basis of a detection result of the detection value variation detecting section during a time period from the start of the switching of the target air-fuel ratio by the target air-fuel ratio switching section. The diagnosis control gain setting section sets a control gain of the air-fuel ratio feedback control section at a value greater than a normal control gain when the diagnosis of the wide-range air-fuel ratio sensor is executed.